Materials for use in electrophoretic displays

ABSTRACT

An electrophoretic medium comprises an electrophoretic layer, a layer of lamination adhesive and a polymeric layer disposed between the electrophoretic layer and the lamination adhesive layer, the polymeric layer being impermeable to the fluid. A second form of electrophoretic medium has a layer of a complex of an alkali metal and a polymer in contact with the electrophoretic layer. A third form of electrophoretic medium comprises a plurality of discrete droplets of internal phase in a binder, and further comprises a salt.

REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 12/905,171,filed Oct. 15, 2010, now U.S. Pat. No. 9,310,661, issued Apr. 12, 2016,which is a division of U.S. application Ser. No. 12/043,472, filed Mar.6, 2008, now U.S. Pat. No. 7,826,129, issued Nov. 2, 2010, which claimsthe benefit of application Ser. No. 60/893,226, filed Mar. 6, 2007.

This application is related to application ser. No. 11/160,364, filedJun. 21, 2005 (Publication No. 2006/0007527, now U.S. Pat. No.7,411,719), which is a continuation-in-part of application Ser. No.10/906,075, filed Feb. 2, 2005 (now U.S. Pat. No. 7,079,305), which isturn is a divisional of application Ser. No. 09/683,903, filed Feb. 28,2002 (now U.S. Pat. No. 6,866,760.

The entire contents of these copending applications, and of all otherU.S. patents and published and copending applications mentioned below,are herein incorporated by reference.

BACKGROUND OF INVENTION

This invention relates to electrophoretic media and displays. Morespecifically, in one aspect this invention relates to an electrophoreticmedium and display having improved mechanical robustness. In anotheraspect, this invention relates to a polymer-dispersed electrophoreticmedium and display with reduced self-erasing.

Electrophoretic displays have been the subject of intense research anddevelopment for a number of years. Such displays can have attributes ofgood brightness and contrast, wide viewing angles, state bistability,and low power consumption when compared with liquid crystal displays.The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. (In practice,some electrophoretic displays, including some of the displays of thepresent invention, are capable of achieving multiple gray states, and,are stable not only in their extreme black and white optical states, butalso in their intermediate gray states. Although such displays shouldproperly be described as “multi-stable” rather than “bistable”, thelatter term may be used herein for convenience.) The optical propertywhich is changed by application of an electric field is typically colorperceptible to the human eye, but may be another optical property, suchas optical transmission, reflectance, luminescence or, in the case ofdisplays intended for machine reading, pseudo-color in the sense of achange in reflectance of electromagnetic wavelengths outside the visiblerange. Nevertheless, problems with the long-term image quality of thesedisplays have prevented their widespread usage. For example, particlesthat make up electrophoretic displays tend to settle, resulting ininadequate service-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. PatentPublication No. 2005/0001810; European Patent Applications 1,462,847;1,482,354; 1,484,635; 1,500,971; 1,501,194; 1,536,271; 1,542,067;1,577,702; 1,577,703; and 1,598,694; and International Applications WO2004/090626; WO 2004/079442; and WO 2004/001498. Such gas-basedelectrophoretic media appear to be susceptible to the same types ofproblems due to particle settling as liquid-based electrophoretic media,when the media are used in an orientation which permits such settling,for example in a sign where the medium is disposed in a vertical plane.Indeed, particle settling appears to be a more serious problem ingas-based electrophoretic media than in liquid-based ones, since thelower viscosity of gaseous suspending fluids as compared with liquidones allows more rapid settling of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporation haverecently been published describing encapsulated electrophoretic media.Such encapsulated media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically-mobileparticles suspended in a liquid suspending medium, and a capsule wallsurrounding the internal phase. Typically, the capsules are themselvesheld within a polymeric binder to form a coherent layer positionedbetween two electrodes. Encapsulated media of this type are described,for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564;6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182;6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949;6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545;6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333;6,704,133; 6,710,540; 6,721,083; 6,724,519; 6,727,881; 6,738,050;6,750,473; 6,753,999; 6,816,147; 6,819,471; 6,822,782; 6,825,068;6,825,829; 6,825,970; 6,831,769; 6,839,158; 6,842,167; 6,842,279;6,842,657; 6,864,875; 6,865,010; 6,866,760; 6,870,661; 6,900,851;6,922,276; 6,950,200; 6,958,848; 6,967,640; 6,982,178; 6,987,603;6,995,550; 7,002,728; 7,012,600; 7,012,735; 7,023,420; 7,030,412;7,030,854; 7,034,783; 7,038,655; 7,061,663; 7,071,913; 7,075,502;7,075,703; 7,079,305; 7,106,296; 7,109,968; 7,110,163; 7,110,164;7,116,318; 7,116,466; 7,119,759; 7,119,772; 7,148,128; 7,167,155;7,170,670; 7,173,752; 7,176,880; 7,180,649; 7,190,008; 7,193,625;7,202,847; 7,202,991; 7,206,119; 7,223,672; 7,230,750; 7,230,751;7,236,790; 7,236,792; 7,242,513; 7,247,379; 7,256,766; 7,259,744;7,280,094; 7,304,634; 7,304,787; 7,312,784; 7,312,794; and 7,312,916;and U.S. Patent Applications Publication Nos. 2002/0060321;2002/0090980; 2003/0102858; 2003/0151702; 2003/0222315; 2004/0105036;2004/0112750; 2004/0119681; 2004/0155857; 2004/0180476; 2004/0190114;2004/0196215; 2004/0226820; 2004/0257635; 2004/0263947; 2005/0000813;2005/0007336; 2005/0012980; 2005/0018273; 2005/0024353; 2005/0062714;2005/0067656; 2005/0099672; 2005/0122284; 2005/0122306; 2005/0122563;2005/0134554; 2005/0151709; 2005/0152018; 2005/0156340; 2005/0179642;2005/0190137; 2005/0212747; 2005/0213191; 2005/0253777; 2005/0280626;2006/0007527; 2006/0038772; 2006/0139308; 2006/0139310; 2006/0139311;2006/0176267; 2006/0181492; 2006/0181504; 2006/0194619; 2006/0197737;2006/0197738; 2006/0202949; 2006/0223282; 2006/0232531; 2006/0245038;2006/0262060; 2006/0279527; 2006/0291034; 2007/0035532; 2007/0035808;2007/0052757; 2007/0057908; 2007/0069247; 2007/0085818; 2007/0091417;2007/0091418; 2007/0097489; 2007/0109219; 2007/0128352; 2007/0146310;2007/0152956; 2007/0153361; 2007/0200795; 2007/0200874; 2007/0201124;2007/0207560; 2007/0211002; 2007/0211331; 2007/0223079; 2007/0247697;2007/0285385; and 2007/0286975; and International ApplicationsPublication Nos. WO 00/38000; WO 00/36560; WO 00/67110; and WO 01/07961;and European Patents Nos. 1,099,207 B 1; and 1,145,072 B 1.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to SipixImaging, Inc.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, theaforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and 5,872,552;6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoreticdisplays, which are similar to electrophoretic displays but rely uponvariations in electric field strength, can operate in a similar mode;see U.S. Pat. No. 4,418,346.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See US Patent Publication No. 2004/0226820);and other similar techniques.) Thus, the resulting display can beflexible. Further, because the display medium can be printed (using avariety of methods), the display itself can be made inexpensively.

An electrophoretic display normally comprises a layer of electrophoreticmaterial and at least two other layers disposed on opposed sides of theelectrophoretic material, one of these two layers being an electrodelayer. In most such displays both the layers are electrode layers, andone or both of the electrode layers are patterned to define the pixelsof the display. For example, one electrode layer may be patterned intoelongate row electrodes and the other into elongate column electrodesrunning at right angles to the row electrodes, the pixels being definedby the intersections of the row and column electrodes. Alternatively,and more commonly, one electrode layer has the form of a singlecontinuous electrode and the other electrode layer is patterned into amatrix of pixel electrodes, each of which defines one pixel of thedisplay. In another type of electrophoretic display, which is intendedfor use with a stylus, print head or similar movable electrode separatefrom the display, only one of the layers adjacent the electrophoreticlayer comprises an electrode, the layer on the opposed side of theelectrophoretic layer typically being a protective layer intended toprevent the movable electrode damaging the electrophoretic layer.

The manufacture of a three-layer electrophoretic display normallyinvolves at least one lamination operation. For example, in several ofthe aforementioned MIT and E Ink patents and applications, there isdescribed a process for manufacturing an encapsulated electrophoreticdisplay in which an encapsulated electrophoretic medium comprisingcapsules in a binder is coated on to a flexible substrate comprisingindium-tin-oxide (ITO) or a similar conductive coating (which acts as anone electrode of the final display) on a plastic film, thecapsules/binder coating being dried to form a coherent layer of theelectrophoretic medium firmly adhered to the substrate. Separately, abackplane, containing an array of pixel electrodes and an appropriatearrangement of conductors to connect the pixel electrodes to drivecircuitry, is prepared. To form the final display, the substrate havingthe capsule/binder layer thereon is laminated to the backplane using alamination adhesive. (A very similar process can be used to prepare anelectrophoretic display usable with a stylus or similar movableelectrode by replacing the backplane with a simple protective layer,such as a plastic film, over which the stylus or other movable electrodecan slide.) In one preferred form of such a process, the backplane isitself flexible and is prepared by printing the pixel electrodes andconductors on a plastic film or other flexible substrate. The obviouslamination technique for mass production of displays by this process isroll lamination using a lamination adhesive.

As discussed in the aforementioned U.S. Pat. No. 6,982,178, many of thecomponents used in electrophoretic displays, and the methods used tomanufacture such displays, are derived from technology used in liquidcrystal displays (LCD's). For example, electrophoretic displays may makeuse of an active matrix backplane comprising an array of transistors ordiodes and a corresponding array of pixel electrodes, and a “continuous”front electrode (in the sense of an electrode which extends overmultiple pixels and typically the whole display) on a transparentsubstrate, these components being essentially the same as in LCD's.However, the methods used for assembling LCD's cannot be used withencapsulated electrophoretic displays. LCD's are normally assembled byforming the backplane and front electrode on separate glass substrates,then adhesively securing these components together leaving a smallaperture between them, placing the resultant assembly under vacuum, andimmersing the assembly in a bath of the liquid crystal, so that theliquid crystal flows through the aperture between the backplane and thefront electrode. Finally, with the liquid crystal in place, the apertureis sealed to provide the final display.

This LCD assembly process cannot readily be transferred to encapsulateddisplays. Because the electrophoretic material is solid, it must bepresent between the backplane and the front electrode before these twointegers are secured to each other. Furthermore, in contrast to a liquidcrystal material, which is simply placed between the front electrode andthe backplane without being attached to either, an encapsulatedelectrophoretic medium normally needs to be secured to both; in mostcases the electrophoretic medium is formed on the front electrode, sincethis is generally easier than forming the medium on thecircuitry-containing backplane, and the front electrode/electrophoreticmedium combination is then laminated to the backplane, typically bycovering the entire surface of the electrophoretic with an adhesive andlaminating under heat, pressure and possibly vacuum.

Electro-optic displays, including electrophoretic displays, can becostly; for example, the cost of the color LCD found in a portablecomputer is typically a substantial fraction of the entire cost of thecomputer. As the use of such displays spreads to devices, such ascellular telephones and personal digital assistants (PDA's), much lesscostly than portable computers, there is great pressure to reduce thecosts of such displays. The ability to form layers of electrophoreticmedia by printing techniques on flexible substrates, as discussed above,opens up the possibility of reducing the cost of electrophoreticcomponents of displays by using mass production techniques such asroll-to-roll coating using commercial equipment used for the productionof coated papers, polymeric films and similar media. However, in thistype of process, it may be necessary to transport the coated medium froma commercial coating plant to the plant used for final assembly ofelectrophoretic displays without damage to the relatively fragile layerof electrophoretic medium.

Also, most prior art methods for final lamination of electrophoreticdisplays are essentially batch methods in which the electrophoreticmedium, the lamination adhesive and the backplane are only broughttogether immediately prior to final assembly, and it is desirable toprovide methods better adapted for mass production.

The aforementioned U.S. Pat. No. 6,982,178 describes a method ofassembling a solid electro-optic display (including an encapsulatedelectrophoretic display) which is well adapted for mass production.Essentially, this patent describes a so-called “front plane laminate”(“FPL”) which comprises, in order, a light-transmissiveelectrically-conductive layer; a layer of a solid electro-optic mediumin electrical contact with the electrically-conductive layer; anadhesive layer; and a release sheet. Typically, the light-transmissiveelectrically-conductive layer will be carried on a light-transmissivesubstrate, which is preferably flexible, in the sense that the substratecan be manually wrapped around a drum (say) 10 inches (254 mm) indiameter without permanent deformation. The term “light-transmissive” isused in this patent and herein to mean that the layer thus designatedtransmits sufficient light to enable an observer, looking through thatlayer, to observe the change in display states of the electro-opticmedium, which will be normally be viewed through theelectrically-conductive layer and adjacent substrate (if present). Thesubstrate will be typically be a polymeric film, and will normally havea thickness in the range of about 1 to about 25 mil (25 to 634 μm),preferably about 2 to about 10 mil (51 to 254 μm). Theelectrically-conductive layer is conveniently a thin metal layer of, forexample, aluminum or ITO, or may be a conductive polymer. Poly(ethyleneterephthalate) (PET) films coated with aluminum or ITO are availablecommercially, for example as “aluminized Mylar” (“Mylar” is a RegisteredTrade Mark) from E.I. du Pont de Nemours & Company, Wilmington Del., andsuch commercial materials may be used with good results in the frontplane laminate.

The aforementioned U.S. Pat. No. 6,982,178 also describes a method fortesting the electro-optic medium in a front plane laminate prior toincorporation of the front plane laminate into a display. In thistesting method, the release sheet is provided with an electricallyconductive layer, and a voltage sufficient to change the optical stateof the electro-optic medium is applied between this electricallyconductive layer and the electrically conductive layer on the opposedside of the electro-optic medium. Observation of the electro-opticmedium will then reveal any faults in the medium, thus avoidinglaminating faulty electro-optic medium into a display, with theresultant cost of scrapping the entire display, not merely the faultyfront plane laminate.

The aforementioned U.S. Pat. No. 6,982,178 also describes a secondmethod for testing the electro-optic medium in a front plane laminate byplacing an electrostatic charge on the release sheet, thus forming animage on the electro-optic medium. This image is then observed in thesame way as before to detect any faults in the electro-optic medium.

The aforementioned 2004/0155857 describes a so-called “double releasefilm” which is essentially a simplified version of the front planelaminate of the aforementioned U.S. Pat. No. 6,982,178. One form of thedouble release sheet comprises a layer of a solid electro-optic mediumsandwiched between two adhesive layers, one or both of the adhesivelayers being covered by a release sheet. Another form of the doublerelease sheet comprises a layer of a solid electro-optic mediumsandwiched between two release sheets. Both forms of the double releasefilm are intended for use in a process generally similar to the processfor assembling an electro-optic display from a front plane laminatealready described, but involving two separate laminations; typically, ina first lamination the double release sheet is laminated to a frontelectrode to form a front sub-assembly, and then in a second laminationthe front sub-assembly is laminated to a backplane to form the finaldisplay, although the order of these two laminations could be reversedif desired.

The aforementioned 2007/0109219 describes a so-called “inverted frontplane laminate”, which is a variant of the front plane laminatedescribed in the aforementioned U.S. Pat. No. 6,982,178. This invertedfront plane laminate comprises, in order, at least one of alight-transmissive protective layer and a light-transmissiveelectrically-conductive layer; an adhesive layer; a layer of a solidelectro-optic medium; and a release sheet. This inverted front planelaminate is used to form an electro-optic display having a layer oflamination adhesive between the electro-optic layer and the frontelectrode or front substrate; a second, typically thin layer of adhesivemay or may not be present between the electro-optic layer and abackplane. Such electro-optic displays can combine good resolution withgood low temperature performance.

The aforementioned 2007/0109219 also describes various methods designedfor high volume manufacture of electro-optic displays using invertedfront plane laminates; preferred forms of these methods are “multi-up”methods designed to allow lamination of components for a plurality ofelectro-optic displays at one time.

Electrophoretic media and displays tend to be mechanically robust, ascompared with, for example, liquid crystal displays, which requiretransparent, typically glass, substrates on both sides of the liquidcrystal medium. Several of the aforementioned E Ink patents andapplications describe processes for producing electrophoretic displaysin which an electrophoretic medium is coated on to a flexible plasticsubstrate provided with an electrically conductive layer, and theresultant electrophoretic medium/substrate sub-assembly is laminated toa backplane containing a matrix of electrodes to form the final display.Furthermore, the aforementioned U.S. Pat. No. 6,825,068 describes abackplane useful in an electrophoretic display and based upon astainless steel foil coated with a polyimide. Such technologies canproduce flexible electrophoretic displays much less susceptible tobreakage than glass-based liquid crystal displays.

However, although electrophoretic displays are mechanically robust, suchdisplays can be damaged under extreme stress, such as may occur when aportable electrophoretic display is dropped or comes into contact with aheavy object, for example in a travel bag. Typically, such failureoccurs by mechanical rupture of the capsule wall, in the case ofcapsule-based displays, or by rupture of the continuous phase inpolymer-dispersed displays. Either type of failure allows the internalphase (the electrophoretic particles and the surrounding fluid) of theelectrophoretic medium to migrate within the display. Typically, alamination adhesive layer is present adjacent the electrophoreticmedium, and the fluid dissolves in this adhesive layer, leaving behindthe electrophoretic particles as an optically inactive, non-switchingarea which causes visual defects in any image thereafter written on thedisplay. Accordingly, there is a need to improve the mechanicalrobustness of electrophoretic media and displays to reduce theoccurrence of such visual defects.

A second aspect of the present invention relates to reducingself-erasing (also called “kickback”) in polymer-dispersedelectrophoretic media and displays. Self-erasing is a phenomenonobserved in some electrophoretic displays (see, for example, Ota, I., etal., “Developments in Electrophoretic Displays”, Proceedings of the SID,18, 243 (1977), where self-erasing was reported in an unencapsulatedelectrophoretic display) whereby, when the voltage applied across thedisplay is switched off, the electrophoretic medium may at leastpartially reverse its optical state, and in some cases a reversevoltage, which may be larger than the operating voltage, can be observedto occur across the electrodes. It appears (although this invention isin no way limited by this belief) that the self-erasing phenomenon isdue to a mismatch in electrical properties between various components ofthe display; in particular, in the case of an encapsulatedelectrophoretic display, it appears that the phenomenon is due to amismatch in electrical properties between the internal phase of themicrocapsules and the polymer layer, namely the microcapsule walls,which is in electrical series with this internal phase. Obviously,self-erasing is highly undesirable in that it reverses (or otherwisedistorts, in the case of a grayscale display) the desired optical stateof the display. It has been found that self-erasing is a particularproblem in polymer-dispersed electrophoretic media and displays, and thepresent invention provides two different approaches to reducingself-erasing in such media and displays.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides an electrophoretic mediumcomprising:

-   -   an electrophoretic layer comprising a plurality of discrete        droplets of an internal phase in a binder, the internal phase        comprising plurality of particles disposed in a fluid, the        particles being capable of moving through the fluid upon        application of an electric field to the medium;    -   a layer of a lamination adhesive; and    -   a polymeric layer disposed between the electrophoretic layer and        the lamination adhesive layer, the polymeric layer being        substantially impermeable to the fluid.

This aspect of the invention may hereinafter be referred to as the“polymeric barrier layer” medium of the invention. The electrophoreticlayer in this medium may be of the capsule-based type, with each of thedroplets of internal phase surrounded by a capsule wall, which is inturn surrounded by a binder. Alternatively, the electrophoretic layermay be of the polymer-dispersed type, with the droplets of internalphase being in direct contact with the binder. This aspect of theinvention may also be useful in microcell electrophoretic media, inwhich the droplets are confined within cavities formed in a polymericbinder medium, which forms the walls of the microcells. The fluid usedmay be liquid or gaseous.

In another aspect, this invention provides a polymer-dispersedelectrophoretic medium comprising an electrophoretic layer comprising aplurality of discrete droplets of an internal phase in a binder, theinternal phase comprising plurality of particles disposed in a fluid,the particles being capable of moving through the fluid upon applicationof an electric field to the medium, the electrophoretic medium furthercomprising a layer of a complex of an alkali metal and a polymer incontact with the electrophoretic layer.

This aspect of the invention may hereinafter be referred to as the“alkali metal polymer complex” medium of the invention. Theelectrophoretic medium may be of the capsule-based type,polymer-dispersed type or microcell type, as previously discussed. Thefluid may be liquid or gaseous.

In another aspect, this invention provides a polymer-dispersedelectrophoretic medium comprising a plurality of discrete droplets of aninternal phase in a binder, the internal phase comprising plurality ofparticles disposed in a fluid, the particles being capable of movingthrough the fluid upon application of an electric field to the medium,the polymer-dispersed electrophoretic medium further comprising a salt.

This aspect of the invention may hereinafter be referred to as the “iondoped” medium of the invention. The salts to be used in such a mediumshould dissolve in the continuous phase of the polymer-dispersedelectrophoretic medium. The cations and anions of the dissolved saltshould be able to separate within the continuous phase, and theseparated ions should be mobile within the continuous phase. In apreferred form of such an ion doped medium, the medium is doped with aquaternary ammonium salt, preferably a tetraalkylammonium salt.

The ion doped medium of the present invention may be of thecapsule-based type, polymer-dispersed type or microcell type, aspreviously discussed. The fluid may be liquid or gaseous.

This invention extends to an electrophoretic display comprising anelectrophoretic medium of the present invention and at least oneelectrode arranged to apply an electric field to the electrophoreticmedium. Such an electrophoretic display may further comprise voltagesupply means arranged to supply voltages to the at least one electrode,the voltage supply means being arranged to drive the electrophoreticmedium to a first optical state in which the medium is lighttransmissive and to a second optical state in which the medium is lightabsorbing. Such an electrophoretic display may further comprise at leastone sheet of light transmissive material disposed adjacent theelectrophoretic medium, so that the electrophoretic display forms avariable transmission window.

This invention also extends to a front plane laminate, double releasefilm, or inverted front plane laminate (all as defined above)incorporating an electrophoretic medium of the present invention.

The displays of the present invention may be used in any application inwhich prior art electrophoretic displays have been used. Thus, forexample, the present displays may be used in electronic book readers,portable computers, tablet computers, cellular telephones, smart cards,signs, watches, shelf labels and flash drives.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the variation of dynamic range andself-erasing against storage time at low relative humidity obtained incertain experiments described in Example 11 below.

FIG. 2 is a graph similar to FIG. 1 but showing corresponding datarelating to storage at high relative humidity, as described in Example11 below.

FIGS. 3 and 4 are graphs similar to FIG. 1 and show respectively thevariation of dynamic range and self-erasing as a function of storagetemperature, as described in Example 11 below.

FIGS. 5 and 6 are graphs similar to those of FIGS. 3 and 4 respectively,but showing the results obtained with storage at high relative humidity,as described in Example 11 below.

FIG. 7 is a graph showing a typical variance of reflectance with timefor a prior art electrophoretic medium of the present invention beingdriven to its black and white extreme optical states, and illustratesthe self-erasing which follows the end of each drive pulse.

FIG. 8 is a graph similar to that of FIG. 7 but showing the reducedself-erasing achieved with an ion doped medium of the present invention.

FIG. 9 is a chart showing the results obtained by using varioustetraalkylammonium salts in ion doped media of the present invention.

DETAILED DESCRIPTION

As already indicated, there are three principal aspects of the presentinvention, namely the polymeric barrier layer medium, the alkali metalpolymer complex medium and the ion doped medium. These three principalaspects will mainly be described separately below, but it should beappreciated that more than one aspect of the present invention may beused in a single electrophoretic medium. For example, a singleelectrophoretic medium could make use of a polymeric barrier layer toprovide improved mechanical robustness and an ion doped medium toprovide reduced self-erasing.

Polymeric Barrier Layer Medium

As already indicated, one principal aspect of the present inventionrelates to electrophoretic media in which a polymeric barrier layer isprovided adjacent the electrophoretic layer, between the electrophoreticlayer and a lamination adhesive layer. The electrophoretic medium may beof the capsule-based or polymer-dispersed type; the invention may alsobe used with microcell electrophoretic media. The polymeric barrierlayer should be substantially impermeable to the fluid present in theinternal phase so that, even if the electrophoretic layer fails bycapsule rupture or rupture of the continuous phase of apolymer-dispersed medium, the polymeric barrier layer will remain intactand maintain a barrier between the electrophoretic layer and thelamination adhesive, thus preventing or reducing the visual defectsdiscussed above. It has also been found that the polymeric barrier layershould be thin and tough.

In a preferred form of such a polymeric barrier layer medium, thepolymeric barrier layer comprises both soft and hard domains. Suchpolymers normally have a glass transition temperature (Tg) below ambienttemperature (i.e., below about 20° C.). Due to their low glasstransition temperature, the polymer chain is flexible and mobile and canabsorb mechanical stress beyond its yield strength. Hard domains serveto increase strength and stiffness of the polymer and thus increasemechanical energy absorption and reduce droplet or capsule deformation.

The hard and soft domains in the polymeric barrier layer may be formedin any known manner, for example by crystallization, cross-linking orthe use of polymer blends. A crystalline polymer usually has amorphousand crystalline regions. The amorphous regions of a crystalline polymerthat has a low Tg are flexible and serve as soft domains whereas thecrystalline regions are hard in nature. The soft and hard regionscombine to give the polymer mechanical toughness so that it is neithertoo brittle nor too soft.

Crosslinking is another way of forming the soft and hard domains in thepolymeric barrier layer. The flexible chains of a low glass transitiontemperature polymer can be linked to form a crosslinked network. Whenthe density of the crosslinks reaches a certain level, a hard domainforms around crosslinking sites.

Some copolymers or polymer blends can also provide soft and harddomains. For example, an AB or ABA type of block copolymer, where blockA has a high glass transition temperature and block B has a low glasstransition temperature, will typically be a thermoplastic containingsoft and hard domains.

The fluids used in most electrophoretic media are hydrocarbon-based. Toprovide a barrier layer impermeable to such fluids, and to reduce costand for environmental reasons, the polymer used in the barrier layerwill typically be a water-soluble polymer. Examples of suitable polymersinclude polyethylene oxide (PEO), polyethylene glycol (PEG),polypropylene oxide, poly(ethylene oxide-co-propylene oxide),poly(ethylene glycol-co-vinyl alcohol), poly(ethylene glycol-co-vinylacetate), poly(ethylene glycol-co-acrylic acid), poly(ethyleneglycol-co-acrylamide), poly(ethylene glycol-co-vinyl pyrrolidone),poly(ethylene glycol-co-methacrylic acid), and poly(ethyleneglycol-co-hydroxyethyl methacrylate). Typically, the barrier polymerwill have a molecular weight in the range of from about 1,000 to about10,000,000, preferably from about 10,000 to about 1,000,000.

Polymeric barrier layer electrophoretic media can readily be prepared bymethods which are well adapted for mass production. Typically, theelectrophoretic medium (in the form of a capsule/binder slurry or anemulsion) is coated on to a substrate and dried. The barrier polymer isthen dissolved in a solvent (typically water) and the resultant polymersolution is applied over the electrophoretic layer on the substrate byspraying, coating or any other convenient technique. The polymericbarrier layer will normally need to be dried, in air or in an oven.Alternatively a dry film of the barrier polymer (alone or coated on arelease sheet) may be laminated to the substrate/electrophoretic layersub-assembly, provided the barrier polymer adheres sufficiently to theelectrophoretic layer and does not damage the electrophoretic layer.Once a substrate/electrophoretic layer/polymeric layer sub-assembly hasbeen produced in this manner, it may be assembled into a display,directly or indirectly, in any known manner. For example, a layer oflamination adhesive can be coated over the polymeric barrier layer andthe resultant sub-assembly laminated to a backplane to form the finaldisplay. Alternatively (and more commonly in industrial practice), alayer of lamination adhesive can be coated on to a release sheet, andthe resultant release sheet/lamination adhesive sub-assembly laminatedto the substrate/electrophoretic layer sub-assembly to form a frontplane laminate, which can later be used in the assembly of anelectrophoretic display. It will readily be apparent to those skilled inthe manufacture of electrophoretic and other electro-optic displays thatthe polymeric barrier layer of the present invention can readily beprovided in a double release film or inverted front plane laminate ofthe present invention by similar techniques.

The following Examples are now given, though by way of illustrationonly, to show preferred materials, conditions and techniques used in thepreparation of polymeric barrier layer media and displays of the presentinvention.

EXAMPLE 1 Control Polymer-dispersed Electrophoretic Display

This Example illustrates the preparation of a control polymer-dispersedelectrophoretic display (i.e., one lacking a polymeric barrier layer) bya process generally similar to that described in the aforementioned U.S.Pat. No. 7,079,305.

Gelatin (6.0 g, supplied by Dynagel, Inc., 10 Wentworth Avenue, CalumetCity, Ill. 60409) was dissolved in deionized water (90 mL) maintained at50° C. in a 500 mL water-jacketed reactor equipped with a mechanicaloverhead stirrer. The reactor was stirred at 100 rpm and an internalphase (90 g) comprising polymer-coated titania particles andpolymer-coated copper chromite particles in a hydrocarbon fluid (thisinternal phase being substantially as described in the paragraphbridging columns 20 and 21 of the aforementioned U.S. Pat. No.7,002,728) was added to the reactor. After the internal phase had becomefully dispersed, poly(vinyl alcohol) (291 g of a 10.3 percent solutionof Kuraray Mowiol 28-99, available from Kuraray Co. Ltd, Tokyo, Japan)was added slowly to the reactor, which was maintained at 50° C. Afterapproximately 15 minutes stirring, the jacket temperature of the reactorwas lowered to 25° C., and thereafter stirring was continued for anadditional 24 hours. After this time, glyoxal (22.5 g of a 40 percentsolution from Aldrich) was added to the reactor with stirring.

The emulsion thus produced was coated using a slot coater with a 190 μmgap on to the ITO-coated surface of a 5 mil (127 μm) poly(ethyleneterephthalate) film coated on one surface with ITO, and the resultantcoating dried to form a polymer-dispersed electrophoretic medium.Separately, a polyurethane adhesive containing diglycidyl aniline as across-linker (see the aforementioned U.S. Pat. No. 7,173,752) was coatedon to a release sheet, and laminated to the emulsion surface of theelectrophoretic medium to form a front plane laminate as described inthe aforementioned U.S. Pat. No. 6,982,178. The release sheet was thenremoved and the remaining layers of the front plane laminate laminatedto the carbon-black coated surface of a PET film coated on one surfacewith carbon black, thus forming experimental 2 inch (51 mm) squareexperimental single pixel electrophoretic displays.

EXAMPLE 2 Polymer-dispersed Electrophoretic Display with Barrier Layer

This Example illustrates the preparation of a polymer-dispersedelectrophoretic display of the present invention having a poly(ethyleneoxide) (PEO) barrier layer by a process similar to that of Example 1.

Example 1 was repeated except that a PEO barrier layer was providedbetween the electrophoretic layer and the lamination adhesive layer. Toprovide this barrier layer, a PEO solution was prepared by dissolvingPEO (7.8 g of a PEO from Aldrich, Mw about 300,000) in deionized water(92.2 mL) and coated over an electrophoretic layer formed as in Example1 above using a bar coater with a wet gap set to 80 μm, resulting in adried PEO layer approximately 7 μm thick. Experimental single pixeldisplays were then prepared as in Example 1 above.

EXAMPLE 3 Control Polymer-dispersed Electrophoretic Display

This Example illustrates the preparation of a control polymer-dispersedelectrophoretic display generally similar to that of Example 1 but usinga different binder.

Gelatin (5.0 g, supplied by Dynagel, Inc.) was dissolved in deionizedwater (50 mL) maintained at 50° C. in a 500 mL water-jacketed reactorequipped with a mechanical overhead stirrer. The reactor was stirred at100 rpm and the same internal phase (120 g) as in Example 1 was added tothe reactor. After the internal phase had become fully dispersed,poly(vinylpyrrolidone) (8.0 g of a 20 percent solution, Mw about1.3×10⁶, from Aldrich) and poly(vinyl alcohol) (310 g of a 9.2 percentsolution of Kuraray Mowiol 28-99) were added slowly to the reactor,which was maintained at 50° C. After approximately 15 minutes stirring,the jacket temperature of the reactor was lowered to 25° C., andthereafter stirring was continued for an additional 24 hours. After thistime, glyoxal (22.5 g of a 40 percent solution) was added to the reactorwith stirring. The emulsion thus produced was then converted toexperimental single pixel displays in the same way as in Example 1above.

EXAMPLE 4 Polymer-dispersed Electrophoretic Display with Barrier Layer

Example 3 was repeated, except that a PEO barrier layer was provided inthe same way as in Example 2, the dried PEO being approximately 5 μmthick.

EXAMPLE 5 Mechanical/Electro-optic Tests

The experimental displays prepared in Examples 1-4 above were driven totheir black and white extreme optical states at ambient temperatureusing 500 millisecond ±15 V voltage pulses with 3 seconds rest betweenpulses. The reflectances of the white and dark extreme optical stateswere measured at the end of the drive pulse and 3 seconds later (thelatter being denoted by “W-3s” and “D-3s” in the Table below); thelatter measurement allows for self-erasing to take place. Thereflectances were converted to L* units (where L* has the usual CIEdefinition:L*=116(R/R ₀)^(1/3)−16,where R is the reflectance and R₀ is a standard reflectance value); theresults are shown in Table 1 below.

TABLE 1 W W-3 s D D-3 s Example (L*) (L*) (L*) (L*) 1 (Control) 68.163.6 25.5 27.4 2 68.0 64.5 26.5 27.8 3 (Control) 72.7 69.4 23.1 24.2 471.8 69.6 25.1 25.5

It will be seen from Table 1 that the provision of the barrier layer inaccordance with the present invention caused little or no change in theelectro-optic properties of the displays.

The experimental displays were then subjected to the followingmechanical tests:

Stylus

This test measures the strength of capsules or droplets (in the case ofa polymer-dispersed electrophoretic medium). Specifically, the test usesa Balanced Beam Scrape Adhesion and Mar tester to impinge a weighted 1mm diameter stylus on a test display. The weight on the stylus issystematically increased until the desired level in increments of 50grams. The load at which 10 or more capsules or droplets beneath thestylus burst is recorded; the figure quoted is the average of threetests.

Ball Drop

This test measures the impact strength of capsules or droplets in anelectrophoretic display. In this test, a stainless ball weighing 2.675grams and having a diameter of 0.25 inch (6.3 mm) is dropped verticallyfrom a controlled height directly on to a test display taped against aflat melamine surface. The impact site is examined using a microscope toassess the damage to the capsules or droplets. The minimum height atwhich the ball causes more than 10 capsules or droplets to burst isrecorded; the figure quoted is the average of three tests.

Hammer

This test also measures the impact strength of capsules or droplets inan electrophoretic display. Specifically, the tester holds an impulsehammer (supplied by Dytran Instruments, Inc. Chatsworth, Calif.) with aflat circular tip 0.25 inch (6.3 mm) in diameter and hits a selectedspot on the test display. The force at which the hammer impacts thedisplay is recorded by a computer. The tester can vary the impact forcemanually. The smallest force that causes 10 or more capsules or dropletsto burst is recorded.

Tapping

This test also measures the strength of capsules or droplets in anelectrophoretic display. The test display is fixed on a horizontal platewith adhesive tape. A cone-shaped pin with a tip diameter of 1 mm is setto tap the device at a frequency of 5 taps per second. The tapping forceis 300 g. The number of taps is recorded by a computer. Periodically,the test is stopped and the test display examined under a microscope.The smallest number of taps that causes 10 or more capsules or dropletsto burst is recorded.

Table 2 below shows the results of these tests for the displays producedin Examples 1 to 4.

TABLE 2 Stylus Ball Drop Hammer Tapping Example (g) (mm)* (lbs. f) (# ofcycles) 1 (Control) 700 200 31.3 15 2 1050 200 38.7 15 3 (Control) 70020 12.0 4 4 1050 30 17.5 10

From Table 2 it will be seen that the provision, in Examples 2 and 4, ofa barrier layer in accordance with the present invention significantlyimproved the mechanical properties of the electrophoretic display; itshould be noted that in the ball drop test, the limit of the testapplied was a 200 mm drop, so the absence of any increase in this valueis not significant. Thus, these tests show that the provision of abarrier layer in accordance with the present invention increases themechanical robustness of the display without sacrificing itselectro-optic performance.

Alkali Metal Polymer Complex Medium

As already indicated, a second principal aspect of the present inventionrelates to reducing self-erasing in polymer-dispersed electrophoreticdisplays by inserting a layer of a complex of an alkali metal and apolymer in contact with the electrophoretic layer.

Polymer-dispersed electrophoretic media have certain advantages overcapsule-based media. For example, the production of polymer-dispersedmedia tends to be simpler because it is only necessary to emulsify theinternal phase in the binder and coat the resultant emulsion.Polymer-dispersed media also tend to have greater mechanical durability.However, polymer-dispersed media tend to suffer from self-erasing,especially under conditions of very low or very high relative humidity.

As mentioned above, self-erasing is a phenomenon whereby someelectrophoretic media spontaneously partially reverse a change inoptical state after the drive pulse which causes the change in opticalstate terminates. For example, if a pixel of a display is driven fromblack to white by a drive pulse, after the drive pulse terminates, thepixel may spontaneously change from the white optical state to a verylight gray. Self-erasing is a rapid phenomenon, and should be carefullydistinguished from the much slower changes in optical state caused bythe finite bistability of electrophoretic and other electro-optic media.In practice, as indicated in Example 5 above, it is convenient tomeasure self-erasing by measuring the reflectance of the electrophoreticmedium at the end of the drive pulse and again 3 seconds later, with thedifference between the two readings (normally expressed in terms of L*units) being the self-erasing. The optical properties of electrophoreticmedia are often not symmetrical between the two extreme optical states,so these two extreme optical states may have different self-erasingvalues.

It has been found that polymer-dispersed electrophoretic media tend todisplay substantial self-erasing in conditions of very low or very highrelative humidity. It is believed (although the invention is in no waylimited by this belief) that the variation of self-erasing with relativehumidity is due to changes in the water content of the polymer-dispersedmedium. For example, in a low humidity environment, water can evaporatefrom the medium, thus causing self-erasing. At high relative humidity,for example 70 percent relative humidity, the medium can absorb waterfrom the air, which changes the electro-optic properties of the mediumand causes high self-erasing.

In accordance with the alkali metal polymer complex aspect of thepresent invention, it has been found that self-erasing inpolymer-dispersed electrophoretic displays can be reduced or eliminatedby inserting a layer of a complex of an alkali metal and a polymer incontact with the electrophoretic layer.

It is known that salts, for example lithium salts, can be dissolved inpolar macromolecular solids to form ionically conducting polymers. Oneexample is such an ionically conducting polymer is that formed bypoly(ethylene oxide) (PEO) and lithium salts. PEO solvates the lithiumcation by coordinating the ion to form a coordination complex, PEO-Li.This complex has been widely used as the key component of lightweightbatteries because of its superb electrical properties.

It will be noted that PEO itself is one of the polymers which may beused as a barrier layer in the polymeric barrier layer medium of thepresent invention. Since the mechanical properties of, for example, thePEO-Li complex, do not differ greatly from those of PEO itself, the useof an alkali metal/polymer complex layer can serve to provide bothreduced self-erasing and improved mechanical robustness to anelectrophoretic medium.

The polymer used to form the alkali metal/polymer complex may be anypolymer electrolyte or polymer which has the structural featuresnecessary for complexing with alkali metals to form polymerelectrolytes. Examples of such polymers are poly(ethylene oxide),poly(dimethyl siloxane), polyoxymethylene,poly(oxymethylene-oxyethylene),poly[bis-(methoxyethoxyethoxy)phosphazene], poly(propylene oxide),polyoxetane, polytetrahydrofuran, and poly(1,3-dioxolane). Typically,the polymer will have a molecular weight in the range of from about1,000 to about 10,000,000, preferably from about 10,000 to about1,000,000. The polymers can be reacted with alkali metal salts to formthe ionically conducting complexes. There are many alkali metal saltsthat may be used for this purpose. The alkali metal used in forming theconductive complexes in this invention may be lithium, sodium orpotassium, but lithium is preferred. Examples of suitable lithium saltsare Li₂SO₄, LiClO₄, LiN(SO₂CF₃)₂, LiSO₃CF₃, and LiBF₄.

The alkali metal/polymer complexes used in the present invention canreadily be prepared by reacting an alkali metal salt with the polymer inorganic or aqueous solution; water is usually the preferred solvent forthis purpose. Typically, in the preferred PEO-Li complex, the molarratio of PEO to lithium will be in the range of about 1 to about 100,preferably about 2 to about 10 and desirably about 3 to about 8.

Incorporation of the alkali metal/polymer complex layer into apolymer-dispersed electrophoretic display can readily be effected usingmethods generally similar to those described above for incorporation ofbarrier layers. The internal phase/binder emulsion is deposited on asubstrate, for example PET/ITO and dried to form a polymer-dispersedelectrophoretic medium. A solution of the alkali metal/polymer complex,typically in water, is coated over the polymer-dispersed electrophoreticlayer by spraying, coating or any other convenient method, and theresult complex-containing layer dried in air or in an oven.Alternatively a dry film of the complex (alone or coated on a releasesheet) may be laminated to the substrate/polymer-dispersedelectrophoretic layer sub-assembly, provided the complex adheressufficiently to the polymer-dispersed electrophoretic layer and does notdamage this electrophoretic layer. Once a substrate/polymer-dispersedelectrophoretic layer/polymer complex layer sub-assembly has beenproduced in this manner, it may be assembled into a display, directly orindirectly, in any known manner, conveniently in the same ways asdescribed above for barrier layer displays. The electro-optical andmechanical properties of the displays can then be measured andevaluated.

The following Examples are now given, though by way of illustrationonly, to show preferred materials, conditions and techniques used in thepreparation of alkali metal polymer complex media and displays of thepresent invention.

EXAMPLE 6 Preparation of Polymer-dispersed Electrophoretic Medium

This Example illustrates the preparation of a polymer-dispersedelectrophoretic medium by a process generally similar to that describedin the aforementioned U.S. Pat. No. 7,079,305.

Gelatin (59.4 g, supplied by Dynagel, Inc.) was dissolved in deionizedwater (297 mL) maintained at 50° C. in a 4 L water-jacketed reactorequipped with a four-blade PTFE stirrer. The reactor was stirred at 120rpm and the same internal phase (495 g) as in Example 1 above was addedto the reactor. After the internal phase had become fully dispersed,poly(vinyl alcohol) (1188 g of a 10 percent solution of Kuraray Mowiol28-99) was added slowly to the reactor, which was maintained at 50° C.After approximately 15 minutes stirring, the jacket temperature of thereactor was lowered to 30° C., and thereafter stirring was continued foran additional 24 hours. After this time, glyoxal (59.4 g of a 40 percentsolution from Aldrich) was added slowly to the reactor with stirring,and the stirring was continued for a further 1 hour.

The emulsion thus produced was coated using a linear coater on to theITO-coated surface of a 5 mil (127 μm) poly(ethylene terephthalate) filmcoated on one surface with ITO to form a polymer-dispersedelectrophoretic medium. The coating weight was 31.8 g m⁻². The resultantpolymer-dispersed electrophoretic medium was converted to experimentalsingle pixel displays in the same way as in Example 1 above.

EXAMPLE 7 Preparation of Poly(Ethylene Oxide) Solution

PEO (40 g, from Aldrich, Mw about 200,000) was dissolved in de-ionizedwater (160 mL) to form a PEO solution having a concentration of 20%.

EXAMPLE 8 Preparation of Poly(Ethylene Oxide)-Lithium Complex

A portion (96 g) of the PEO solution prepared in Example 6 was mixedwith lithium sulfate (2.0 g, from Aldrich) in a 500 mL bottle, which wasplaced on a rolling mill for 4 hours to form the desired complex.

EXAMPLE 9 Preparation of Control/PEO Display

The simple PEO solution prepared in Example 7 was coated over thepolymer-dispersed electrophoretic layer prepared in Example 6 using abar coater with wet gap set to 30 to 80 μm, and the resultant film driedin air to produce a dried PEO layer having a thickness of about 5 to 10μm. The resultant polymer-dispersed electrophoretic medium was convertedto experimental single pixel displays in the same way as in Example 1above.

EXAMPLE 10 Preparation of Alkali Metal Polymer Complex Display

Example 9 was repeated, except that the PEO-Li complex solution preparedin Example 8 was used in place of the simple PEO solution prepared inExample 7. Again, the resultant polymer-dispersed electrophoretic mediumwas converted to experimental single pixel displays in the same way asin Example 1 above.

EXAMPLE 11 Electro-optic Measurements

The experimental displays produced in Examples 6, 9 and 10 above weretested for their electro-optic performance under varying conditions.Since the displays of Examples 6 and 9 are both control displays, thesedisplays are hereinafter referred to as the “Control/none” and the“Control/PEO” respectively, while the Example 10 display of the presentinvention is hereinafter referred to as the “PEO-Li” display. Thedriving method used was the same as in Example 5 above.

In a first series of tests, the experimental displays produced inExamples 6, 9 and 10 above, as originally prepared, were driven to theirblack and white extreme optical states and measured for self-erasing inthe same way as in Example 5 above. The results are shown in Table 3below, which shows the total self-erasing, i.e., the sum of the dark andwhite state self-erasing values.

TABLE 3 W W-3s D D-3s Total SE Example Description (L*) (L*) (L*) (L*)(L*) 6 Control/none 73.8 70.8 22.1 26.0 6.9 9 Control/PEO 74.1 69.6 23.027.8 9.3 10 PEO-Li 73.4 71.4 22.0 21.8 2.2

The data in Table 3 show that the presence of the PEO-Li layereffectively reduced the total self-erasing, whereas the simple PEO layerwithout lithium did not do so. Since the PEO-Li and simple PEO layersshould have essentially the same effectiveness as humectants, theresults in Table 3 indicate that the effectiveness of the PEO-Li layerin reducing self-erasing is not due to simple activity as a humectant.

In the next series of tests, the displays of Examples 6 and 10 werestored at ambient temperature but at 20 percent relative humidity forseveral days, and data corresponding to those shown in Table 3 collectedat intervals. FIG. 1 of the accompanying drawings plots the dynamicrange (i.e., the difference between the L* values of the white and darkextreme optical states, measured at the end of the drive pulse) and thetotal self-erasing for the Control Example 6 display and for the PEO-Lidisplay of Example 10. From FIG. 1, it will be seen that the dynamicrange of the Control display decreased rapidly on long term storage atthis low humidity, whereas the dynamic range of the PEO-Li display ofthe present invention declined much more slowly. Furthermore, the totalself-erasing of the Control display increased rapidly, whereas the totalself-erasing of the PEO-Li display of the present invention wasessentially constant.

These tests were then repeated but with storage at 70 percent relativehumidity instead of 20, and the results are shown in FIG. 2 of theaccompanying drawings. From this Figure, it will be seen that bothdisplays maintained large dynamic ranges over the whole test period;although the dynamic range of the PEO-Li display of the presentinvention declined slightly, it remained about 45 L*, which isacceptable for practical purposes. However, the Control display showed ahigh, substantially constant total self-erasing of about 10 L*throughout the test period, whereas the total self-erasing of the PEO-Lidisplay of the present invention initially decreased, presumably as aresult of “conditioning” of the display in the humid environment, andthereafter remained low and constant.

The displays of Example 6 and 10 were also tested in the same way as inthe first series of tests described above (i.e., at ambient humidity)but at differing temperatures. FIG. 3 of the accompanying drawings showsthe variation of the dynamic range of the two displays with temperature,while FIG. 4 shows the corresponding variation of total self-erasing.From FIG. 3, it will be seen that, although the dynamic range of bothdisplays decreased substantially with temperature, the PEO-Li display ofthe present invention decreased more slowly. Using a reasonableassumption that a dynamic range of about 30 L* provides aless-than-optimum but useable image for many purposes, the display ofthe present invention would be useable down to about −25° C., whereasthe Control display would not be useable below about 8° C. From FIG. 4,it will be seen that the display of the present invention hassubstantially lower total self-erasing over the entire temperature rangetested, the difference being especially large over the temperature rangeof about 10 to about −20° C.

Previous studies by E Ink workers had found that the electro-opticperformance of polymer-dispersed electrophoretic displays is veryadversely affected at high and low temperatures after the displays hadbeen exposed to a high humidity environment for several days and thushad presumably absorbed large amounts of moisture. Accordingly, thedisplays of Examples 6 and 10 were stored in a 70 per relative humiditychamber for 6 days, and their dynamic ranges (measured in this caseafter the 3 second rest period and thus after allowing self-erasing tooccur) and total self-erasing values measured over the same temperatureranges as in the preceding series of tests. The results are shown inFIGS. 5 and 6 of the accompanying drawings.

From FIG. 5, it will be seen that the display of the present inventionhad slightly better dynamic range than the Control display at theextremes of the temperature range tested; the PEO-Li display isapproximately 3 L* better at both −30 and +60° C. More importantly, FIG.6 shows that the total self-erasing of the display of the presentinvention remained acceptably low throughout the temperature rangetested, not exceeding about 5 L*, whereas the total self-erasing of theControl display was totally unacceptable at the extremes of thetemperature range.

From the foregoing, it will be seen that incorporation of an alkalimetal/polymer complex layer into a polymer-dispersed electrophoreticdisplay in accordance with the present invention substantially improvesthe electro-optic properties of the display. Total self-erasing isreduced, especially when the display is exposed to very dry or wetenvironments. Furthermore, the total self-erasing is still acceptablewhen the display is exposed to high or low temperatures, even afterexposure to a wet environment. The use of an alkali metal/polymercomplex layer may also be useful in capsule-based electrophoreticdisplays.

Ion Doped Medium

As already mentioned, in its third principal aspect this inventionprovides a polymer-dispersed electrophoretic medium doped with a salt.

As discussed above, although polymer-dispersed electrophoretic mediapossess certain advantages over other types of electrophoretic media,polymer-dispersed media tend to suffer from self-erasing, especiallyunder conditions of very low or very high relative humidity. It has nowbeen found that simple addition of dissolved salts to the emulsion usedto prepare polymer-dispersed media can dramatically improve self-erasingin polymer-dispersed media, may reduce the time necessary to conditionsuch media (i.e., reduce the time such media have to be stored undercontrolled conditions to achieve stable electro-optic properties), andmay improve moisture tolerance.

FIG. 7 of the accompanying drawings shows a typical electro-opticresponse for a polymer-dispersed electrophoretic display driven insubstantially the same manner as in Example 5 above, with a drive pulsein one direction, followed by a 3 second rest period (in which novoltage is applied across the display), a drive pulse in the oppositedirection and another 3 second rest pulse. As indicated in FIG. 7, thetrace shows significant self-erasing following the end of each drivepulse. To an observer of such a display, the self-erasing appears as anoticeable flicker on switching of the display.

It is believed (although the invention is in no way limited by thisbelief) that self-erasing may be caused, at least in part, by a mismatchin conductivity between the droplets and the continuous phase of apolymer-dispersed electrophoretic medium, and hence that incorporatingsalts into the continuous phase can reduce self-erasing by balancing theconductivities of the two phases. It will be evident to those skilled inthe technologies of electro-optic displays that to be effective inincreasing the ionic conductivities of the continuous phase that saltused must have the characteristics recited above, i.e., the salt must bereasonable soluble in the continuous phase, the cation and anion of thesalt must be able to separate in the continuous phase, and the separatedions must be mobile in the continuous phase.

FIG. 8 of the accompanying drawings shows a trace similar to that ofFIG. 7 but taken after additional of a small amount of the salttetrabutylammonium chloride to the emulsion used to prepare the display,the emulsion being otherwise unchanged. By comparing FIG. 8 with FIG. 7,it will be seen that addition of the salt substantially lowers theself-erasing. The exact data plotted in FIGS. 7 and 8 are as shown inTable 4 below (“WSSE” is white state self-erasing, “DSSE” is dark stateself-erasing and “TSSE” is total self-erasing).

TABLE 4 Addi- tive WS WS-3s WSSE DS DS-3s DSSE DR TSSE No 70.0 64.2 5.823.1 26.5 3.4 37.7 9.2 Salt Salt 65.3 64.4 0.9 28.0 27.5 0.5 36.9 1.4

Thus, addition of the salt reduced the total self in this case from 9.2L* to 1.4 L*.

FIG. 9 of the accompanying drawings is a chart showing the resultsobtained on adding various tetraalkylammonium salts to a poly(vinylalcohol) based polymer-dispersed electrophoretic display, the materialsbeing added in a ratio of 25 mmole/Kg of poly(vinyl alcohol), ascompared to the display with no salt additive. Tetrasubstituted ammoniumsalts, especially tetraalkylammonium salts, are highly effective inreducing self-erasing; specific preferred salts includetetrabutylammonium halides, sulfates and hexafluorophosphates. Usefulamounts of the salt were found to be from about 10 to about 1000millimoles of additive per kilogram of binder, with the optimum amountusually being with the range of from about 10 to about 25 millimoles ofadditive per kilogram of binder. These salts are non-toxic and onlysmall quantities are required. However, the concentration of saltrequired can be reduced by controlling the particle size of the salt, asdescribed below.

The salts and other ionic species used in the present invention canreadily be incorporated into the emulsions used to preparedpolymer-dispersed electrophoretic displays. In a typical procedure, toan emulsion of an internal phase in a poly(vinyl alcohol) based binderis added a sufficient quantity of a 0.1 mmolar aqueous solution oftetrabutylammonium chloride to give a salt concentration of 25 mmole perkilogram of poly(vinyl alcohol). The salt is allowed to mix with theemulsion for at least 15 minutes, and the emulsion is then coated, driedand formed into the final display.

It has been found important to control the size of the salt crystalsadded to the electrophoretic medium. The preferred tetrabutylammoniumhexafluorophosphate (TBAHFP) salt is extremely hydrophobic and tends toflocculate in water and aqueous media rather than dispersing ordissolving. Since liquids used in polymer-dispersed electrophoreticmedia are typically hydrocarbon-based, as noted above, the polymers usedin the continuous phase are typically hydrophilic, and may include forexample gelatin and poly(vinyl alcohol). It has been found advantageousto include polyvinylpyrrolidone (PVP) in the continuous phase when usingthe preferred TBAHFP salt, since the PVP acts as a surface active agentand assists in dispersing the TBAHFP throughout the continuous phase;the PVP molecules keep the salt crystals dispersed in the continuousphase and prevent them from flocculating.

Typical commercial forms of TBAHFP have crystal sizes in the range ofabout 100 to about 1000 μm. Adding such large crystals directly to thecontinuous phase results in ineffective use of the salt, with the needto include a relatively large proportion of the salt (around 10-25mmole/Kg of continuous phase), which (it has been found) adverselyaffects the dynamic range of the electrophoretic medium, although itdoes reduce self-erasing. Furthermore, since the droplets of internalphase present in the electrophoretic medium are typically of the orderof 30-50 μm, the salt crystals are substantially larger than thedroplets are the crystals themselves can give rise to noticeable visualdefects in coated films.

It has been found that, by grinding TBAHFP (or other salt additive) downto an average crystal size below about 5 μm, and preferably around 4 μm,the amount of salt required is reduced, visual defects in the coatedfilms are avoided, and the self-erasing experienced by theelectrophoretic medium is reduced without any negative effect on itsdynamic range. The necessary reduction in crystal size can readily beeffected using conventional apparatus. For example, one specific processwhich has been found to give good results is mixing TBAHFP (50 g) with aPVP solution (75 g of a 1 percent solution) in an attritor, and adding250 mL of 2 mm zirconium beads. The attritor is then operated at 600 rpmfor one hour, then turned off and its contents poured into anothercontainer, where the dispersion is vacuum-filtered to separate it fromthe grinding beads. Following filtration, the dispersion is made morefluid by adding an equal volume of additional 1 percent PVP solution,thus reducing the salt concentration from 40 to 20 percent. Followingthis grinding and dilution procedure, it was found that the average sizeof the TBAHFP crystals had been reduced to about 4 μm Such smallcrystals are an order of magnitude smaller than the droplets of internalphase and do not give rise to visible defects in coated films; it isbelieved (although the invention is in no way limited by this belief)that such small crystals can lie concealed in the interstitial spacesbetween packed internal phase droplets in a coated film to theelectrophoretic medium. The salt is conveniently added simultaneouslywith the cross-linker, such a glyoxal, which is typically added at theend of the emulsification process, as described in the patents andapplication mentioned in the Reference to Related Applications Sectionabove.

It has been found that, with the average crystal size reduced by about 4μm by the procedure described above, the concentration of TBAHFP neededto achieve substantial reduction in self-erasing without significanteffect on dynamic range can be greatly reduced; salt concentrations inthe range of about 5 to 500 μmole of TBAHFP per kilogram of poly(vinylalcohol) (measured on a solids basis) have been found to give goodresults, with the optimum salt concentration typically being about 50μmole of TBAHFP per kilogram of poly(vinyl alcohol).

EXAMPLE 12 Effect of Salts on Dynamic Range and Self-erasing

Tetrabutylammonium chloride, iodide and hexafluorophosphate were addedto a poly(vinyl alcohol) based emulsion containing essentially the sameinternal phase as used in Example 1 above at a concentration of 25mmole/Kg of poly(vinyl alcohol), and the resultant emulsions coated,dried and formed into displays substantially as described in Example 6above. Control displays were also prepared with no salt additive.Separate sets of displays thus produced were placed in 30 and 80 percentrelative humidity environments and the dynamic range and totalself-erasing of the displays were measured in the same way as in Example11 above. The results are shown in Table 5 below; all data are in L*units.

TABLE 5 Additive None NBu₄Cl NBu₄I NBu₄PF₆ RH - Days DR TSSE DR TSSE DRTSSE DR TSSE 30% - 0 Days 40.6 6.2 35.9 3.1 31.8 1.5 39.5 2.4 30% - 1Days 36.5 4.1 37.4 3.8 32.3 1.9 36.2 2.5 30% - 2 Days 36.1 5.4 36.7 4.729.2 3.3 36.4 4.2 80% - 0 Days 40.6 6.2 35.9 3.1 31.8 1.5 39.5 2.4 80% -1 Days 33.4 6.8 38.8 3.7 33.1 1.7 40.4 3.1 80% - 2 Days 29.5 8.6 33.02.0 27.4 2.5 39.9 4.1

As noted above, polymer-dispersed electrophoretic displays tend to showincreased self-erasing in very dry or moist environments, thus reducingthe dynamic range of the displays. Addition of salts may provide abarrier to moisture ingress or egress, thus improving the humiditytolerance of the polymer-dispersed electrophoretic displays. The controldisplays included in Table 5 show this behavior; note especially therapid decrease in the dynamic range of the control display at 80 percentrelative humidity, and the parallel increase in total self-erasing. Allof the displays of the invention reduce the loss in dynamic range, withthe tetrabutylammonium hexafluorophosphate essentially preventing anyloss of dynamic range at all. All of the displays of the invention alsosignificantly reduced the total self-erasing, as compared with thecontrols; note that in no case did the total self-erasing of a displayof the invention exceed 5 L*.

The alkali metal/polymer complex and ion doped medium aspects of thepresent invention are both intended to reduce self-erasing, and hencemaintain dynamic range in polymer-dispersed electrophoretic displays.These two aspects of the present invention may be combined in a singledisplay, and may provide better results than either approach alone.

Numerous changes and modifications can be made in the preferredembodiments of the present invention already described without departingfrom the scope of the invention. Accordingly, the foregoing descriptionis to be construed in an illustrative and not in a limitative sense.

The invention claimed is:
 1. A polymer-dispersed electrophoretic mediumassembly comprising: an electrophoretic layer comprising a plurality ofdiscrete droplets of an internal phase in a binder, the internal phasecomprising a plurality of particles disposed in a fluid, the particlesbeing capable of moving through the fluid upon application of anelectric field to the medium, wherein the droplets of internal phase arein direct contact with the binder; and a layer comprising a complex ofpoly(ethylene oxide) and lithium, the layer comprising the complex beingin contact with the electrophoretic layer.
 2. The electrophoretic mediumassembly according to claim 1 wherein the polymer has a molecular weightin the range of from about 1,000 to 10,000,000.
 3. The electrophoreticmedium assembly according to claim 2 wherein the polymer has a molecularweight in the range of from about 10,000 to 1,000,000.
 4. Theelectrophoretic medium assembly according to claim 1, wherein the molarratio of poly(ethylene oxide) to lithium is in the range of 2 to
 10. 5.An electrophoretic display comprising an electrophoretic medium assemblyaccording to claim 1 and at least one electrode arranged to apply anelectric field to the electrophoretic medium.
 6. A front plane laminatecomprising in order: a light-transmissive electrically-conductive layer;a layer of a solid electrophoretic medium in electrical contact with theelectrically-conductive layer; an adhesive layer; and a release sheet,wherein the electrophoretic medium comprises a medium assembly accordingto claim
 1. 7. A double release film comprising: a layer of a solidelectrophoretic medium comprising a binder and having first and secondsurfaces on opposed sides thereof; a first adhesive layer on the firstsurface of the layer of electro-optic medium; a release sheet disposedon the opposed side of the first adhesive layer from the layer ofelectro-optic medium; and a second adhesive layer on the second surfaceof the layer of electro-optic medium, wherein the electrophoretic mediumcomprises a medium assembly according to claim
 1. 8. An inverted frontplane laminate for use in forming an electrophoretic display, thearticle of manufacture comprising, in order: at least one of alight-transmissive protective layer and a light-transmissiveelectrically-conductive layer; an adhesive layer; a layer of a solidelectrophoretic medium comprising a binder; and a release sheet, whereinthe electrophoretic medium comprises an electrophoretic medium assemblyaccording to claim
 1. 9. An electronic book reader, portable computer,tablet computer, cellular telephone, smart card, sign, watch, shelflabel or flash drive comprising a display according to claim 5.